FIG. 1 partially and schematically shows two photodiodes side by side of a network of photodiodes, for example forming an image sensor. Each photodiode comprises a semiconductor area 1 for converting photons into electron-hole pairs. A complete photodiode comprises junctions (not shown) between semiconductor regions of opposite type to store photons, and various read transistors to transfer electrons.
PCT patent application WO2012/032495 (incorporated by reference) discloses that the introduction of light into the photodiode raises an issue when the lateral dimensions of the illuminated surface (the upper surface in FIG. 1) of a photodiode are very small, in the order of wavelength λ of the light that the photodiode is intended to capture, or operating wavelength. Thus, such photodiodes of very small dimensions have a low quantum efficiency. The PCT patent application provides, to increase the quantum efficiency of the photodiode, arranging on the upper surface thereof a pad 2 having lateral dimensions much smaller than the lateral dimensions of the photodiode.
FIG. 2 partially and schematically shows the detection portion of a single-photon avalanche diode, currently called SPAD. Such a diode comprises a structure formed of an N-type semiconductor layer 10 pinned between two semiconductor layers 12 and 13 of opposite type. The problem is that layer 10 is, in modern technologies, very thin, with a thickness which does not exceed from 1 to 1.5 μm. Layer 10 is where the useful conversion of photons into electron-hole pairs is to be performed, while it is known that, in the case of silicon and for an infrared radiation, the layer where the electron-hole pairs are to be created should have a thickness greater than 10 μm, to expect a photon conversion rate greater than 90%. Thus, the efficiency of a SPAD diode manufactured with current technologies does not exceed from 5 to 7%. To improve this efficiency and to avoid losing reflected light, upper semiconductor layer 12 has an antireflection structure, alternately comprising at least one layer of material of low index 14, for example, silicon oxide, and one layer of material of higher index 15, for example, silicon nitride, arranged thereon. Upper protection layer 16 currently is a silicon oxide layer.
Thus, a problem arises to absorb the maximum possible number of photons in structures with pixels of small dimensions such as shown in FIG. 1, and in structures where the layer of photon conversion into electron-hole pairs is particularly thin, such as the SPAD structure of FIG. 2. More generally, this problem arises more or less in all semiconductor photodiodes.
It should be noted that in photodiodes of very small dimensions or of SPAD type, an increase, even low, of the quantum efficiency or absorption rate of the useful portion of the photodiode is in practice extremely important for the detection of light of low intensity. Thus, an efficiency gain from 1 to 5% will be considered as a very significant gain by the user.